The elusive phenylethynyl radical and its cation: synthesis, electronic structure, and reactivity

Abstract

Alkynyl radicals and cations are crucial reactive intermediates in chemistry, but often evade direct detection. Herein, we report the direct observation of the phenylethynyl radical (C₆H₅CC˙) and its cation (C₆H₅CC⁺), which are two of the most reactive intermediates in organic chemistry. The radical is generated via pyrolysis of (bromoethynyl)benzene at temperatures above 1500 K and is characterized by photoion mass-selected threshold photoelectron spectroscopy (ms-TPES). Photoionization of the phenylethynyl radical yields the phenylethynyl cation, which has never been synthesized due to its extreme electrophilicity. Vibrationally-resolved ms-TPES assisted by ab initio calculations unveiled the complex electronic structure of the phenylethynyl cation, which appears at an adiabatic ionization energy (AIE) of 8.90 ± 0.05 eV and exhibits an uncommon triplet (³B₁) ground state, while the closed-shell singlet (¹A₁) state lies just 2.8 kcal mol⁻¹ (0.12 eV) higher in energy. The reactive phenylethynyl radical abstracts hydrogen to form ethynylbenzene (C₆H₅CCH) but also isomerizes via H-shift to the o-, m-, and p-ethynylphenyl isomers (C₆H₄CCH). These radicals are very reactive and undergo ring-opening followed by H-loss to form a mixture of C₈H₄ triynes, along with low yields of cyclic 3- and 4-ethynylbenzynes (C₆H₃CCH). At higher temperatures, dehydrogenation from the unbranched C₈H₄ triynes forms the linear tetraacetylene (C₈H₂), an astrochemically relevant polyyne.